During malignant transformation, cells acquire a motile, invasive phenotype. Rho GTPases play a central role in this process, mediated through their effects on actin remodeling. We recently identified TRE17 as a novel effector for the Rho GTPase, Cdc42. Overexpression of TRE17 induces formation of actin-based filopodial projections, phenocopying Cdc42. Preliminary work indicates that these projections promote degradation of the extracellular matrix (ECM). TRE17 was recently identified as a target of chromosomal translocation in an invasive bone tumor called aneurysmal bone cyst (ABC). Rearrangement resulted in high expression of full length TRE17. High expression of TRE17 has also been detected in Ewing's sarcoma, a malignant and invasive neoplasm. These observations lead us to hypothesize that TRE17 promotes motile, invasive behavior through its effects on actin remodeling and ECM degradation. The goal of my thesis is to determine TRE17's function in the transformed phenotype of Ewing's sarcoma, and to elucidate its mechanism of action. This will be addressed as follows: Aim I: We will (1) identify Ewing sarcoma cell lines which express TRE17;(2) generate derivatives of these cell lines in which TRE17 expression is knocked down by siRNA;and (3) generate "rescue" cell lines expressing distinct TRE17 isoforms and mutants in order to identify functionally important domains. Aim II: To identify which aspects of transformation TRE17 regulates, using the cell lines generated in Aim I we will monitor cell proliferation, anchorage-independent growth, extracellular matrix degradation, and cell motility and invasion. Aim III: Preliminary studies identify IQGAP1 as a direct binding partner for TRE17. IQGAP1 promotes motile, invasive behavior, leading us to hypothesize that TRE17's function in Ewing's sarcoma is dependent on its interaction with IQGAP1. This will be addressed by testing whether the transformation properties identified in Aim II require TRE17's interaction with IQGAP1. Multiple approaches will be utilized, including expression of dominant negative mutants and siRNA-mediated knockdown of IQGAP1. The molecular functions of TRE17 in human neoplasia are poorly understood. However, its expression pattern in human tumors suggests that it may play a unique role in the development of solid mesenchymal tumors, which are greatly understudied in comparison to epithelial cancers. Our work may thus provide novel insights into the mechanisms underlying the invasive, metastatic behavior of Ewing's sarcoma, one of the most common and lethal of this class of neoplasms.